Method and system for embedding and detecting a pattern

ABSTRACT

A method of embedding a pattern as a watermark into a content segment. Prior to modifying the content segment, an impulse response of a filter to be used for detecting the pattern is determined; the time-reversed impulse response of the filter is inserted into the segment as the set of imperceptible features; wherein the filter is an infinite impulse response filter having a semi-white frequency spectrum and provides a pseudo-random time-domain response.

PRIORITY CLAIM

This patent application is a U.S. National Phase of International PatentApplication No. PCT/NL2013/050201, filed 18 Mar. 2013, which claimspriority to Dutch Patent Application No. 2008511, filed 21 Mar. 2012,and U.S. Provisional Application No. 61/613,562, filed 21 Mar. 2012, thedisclosures of which are incorporated herein by reference in theirentirety.

FIELD OF THE INVENTION

The invention relates to a method of embedding a pattern as a watermarkinto a content segment, comprising modifying the content segment toinsert a set of substantially imperceptible features that relate to thepattern.

The invention further relates to a method of detecting a patternrepresenting a payload 10 embedded as a watermark into a contentsegment, comprising detecting a set of substantially imperceptiblefeatures that relate to the pattern in the content segment.

The invention further relates to systems for implementing the methods ofthe invention and to computer program products for causing a computer toexecute the methods of the 15 invention.

BACKGROUND OF THE INVENTION

Watermarking is a technique whereby a payload is represented as one ormore watermark 20 patterns that are subsequently embedded in contentsuch as movies, pictures, songs, or radio and television shows. Thecontent is then distributed e.g. by broadcast, streaming, downloading orsimilar technique or on a storage medium such as a CD, DVD, Blu-Raydisc, flash memory or hard disk. The embedding is usually imperceptible:a person who views or hears the content generally will be unable to pickup the embedded pattern. In some cases, 25 some small parts of theembedded patterns may be perceptible, e.g. as minor degradations inaudio or video quality. This pattern is typically a (pseudo)randompattern, although this may depend on the application and technique forembedding and detecting. However, specialized detection tools canreconstruct the pattern and recover the payload from the reconstruction.Watermarking has many applications, for example tracing the source and30 distribution path of content items, audience measurement,synchronization of media streams, enriching content with metadata orproviding a basis for digital rights management.

A watermarking application consists of two stages. The first is anembedder, which adds a watermark to the content. During this embeddingstage, a certain message or an amount of 35 information (often referredto as ‘payload’) is represented as a prescribed set of changes that areapplied to the content. The second stage is a detector, which extractsthe information that was embedded in the content.

One important class of watermarking techniques is referred to asspread-spectrum watermarking. A characteristic property ofspread-spectrum watermarking methods is that a relatively small amountof information (the payload) is represented by a sequence of patternswhich often have a large bandwidth (e.g., the embedded pattern comprisesnon-zero components across a wide range of signal frequencies), suchthat the associated watermark energy density can be very small. In anadditive spread-spectrum audio watermarking application, a payload isrepresented by two or more pseudo-random patterns that are added (bysummation) to the audio (host) signal.

Known additive watermarking algorithms embed patterns that are detectedusing a matched filter, comprising correlation with a set of candidatepatterns. The advantages of such methods are their virtually unlimitedset of independent patterns that can be embedded, and the flexibility interms of pattern properties (size, bandwidth, etc). A disadvantage isthat detection is in most cases associated with a significantcomputational complexity due to the correlation operation. Moreover, asthis correlation operation is performed most efficiently in a block-wisemanner using a discrete Fourier transform, synchronization of analysisblocks and the watermarks embedded in the audio content is crucial forgood detection performance.

An alternative to additive watermarking is phase modulationwatermarking. Instead of embedding a pattern by addition of the patternto the content segment, phase modulation modifies the phase ofindividual frequency components of the content segment according to thephase of the frequency components of the pattern. This method isdescribed in more detail in M. Arnold, P. Baum, and W. Voessing. “Aphase modulation audio watermarking technique”. In S. Katzenbeisser andA.-R. Sadeghi, editors, Information Hiding, volume 5806 of Lecture Notesin Computer Science, pages 102-116. Springer Berlin/Heidelberg, 2009.Just like in the additive embedding method, the detection process ofthis phase modulation method constitutes correlation with a set ofcandidate patterns.

SUMMARY OF THE INVENTION

An object of the invention is to provide a more efficient watermarkingembedding and detection implementation.

The invention achieves this object by a method of and system forembedding which is characterized in that prior to modifying the contentsegment, an impulse response of a filter to be used for detecting thepattern is determined; the time-reversed impulse response of the filteris inserted into the segment as the set of imperceptible features;wherein the filter is an infinite impulse response filter. In anembodiment, the filter can be characterized by a semi-white frequencyspectrum and provides a pseudo-random time-domain response.

The invention further achieves this object by a method of and system fordetecting which is characterized in that a filter is applied to thecontent segment, the filter being an infinite impulse response filter.In an embodiment, the filter can be characterized by a semi-whitefrequency spectrum and providing a pseudo-random time response, and theoutput of the filter is processed to obtain the pattern from which thepayload can potentially be reconstructed.

With the invention, one embeds a pattern that has specificcharacteristics. This pattern changes sequentially, that is differentpatterns will be embedded in different audio segments. The order ofthese patterns relates to the payload, thus effectively achieving anembedding of the payload as a watermark. Various advantageousembodiments of constructing the pattern are elaborated upon below. Theinvention is based on the insight that the correlation operationnormally used in watermarking has similarities with the convolutionoperation of applying an finite-impulse response onto an input signal toobtain a filtered signal.

A watermark detector correlates candidate watermark sequences, denotedas w_(i)[n], with watermarked content to determine the most likelysequence. The detector typically comprises a matched-filter for a set ofcandidate watermark patterns. For one or more candidate patterns i, eachof the associated sequence of pseudo-random patterns w_(i)[n] iscorrelated with the input signal y[n] as

$\begin{matrix}{r_{i} = {\max\limits_{n}\mspace{11mu}{\sum\limits_{t}\;{{y\left\lbrack {x + n} \right\rbrack}{w_{i}\lbrack t\rbrack}}}}} & {{Eq}.\mspace{14mu} 1}\end{matrix}$where r_(i) denotes the correlation between pattern w_(i) and signal.

In a convolution operation one applies a finite-impulse response h[t]onto an input signal y[n] to obtain a filtered signal z[n]:

$\begin{matrix}{{z\lbrack n\rbrack} = {\sum\limits_{t}\;{{y\left\lbrack {n - t} \right\rbrack}{h\lbrack t\rbrack}}}} & {{Eq}.\mspace{14mu} 2}\end{matrix}$

This equation can be re-formulated as:

$\begin{matrix}{{z\lbrack n\rbrack} = {\sum\limits_{t}\;{{y\left\lbrack {n + t} \right\rbrack}{h\left\lbrack {- t} \right\rbrack}}}} & {{Eq}.\mspace{14mu} 3}\end{matrix}$

When one substitutes h[−t]=w_(i)[t] the result is identical to acorrelation operation as described above. Thus, one can use a(convolution) operation as detector if the time-reversed impulseresponse of that filter is used in the embedding stage. Moreimportantly, besides the use of convolution (which represents afiltering operation with a finite impulse response), the filteroperation at the detector may also be implemented as a filter structurewith feedback loops, e.g. an infinite impulse response (IIR) filter, aslong as the time inverse of the impulse response of the filter wasembedded. This insight is new and not suggested by the prior art.

The article ‘A new Wiener filtering based detection scheme for timedomain perceptual audio watermarking’ by S. Larbi, M. Jaïdane, N. Moreau(IEEE International Conference on Acoustics, Speech and SignalProcessing (ICASSP), Montreal, Canada, 2004, 5:949-952) disclosesdetection method for a spread spectrum and perceptual watermarkingsystem that employs a Wiener de-convolution filter. The paper disclosesapplying spectral shaping of a watermark sequence in the embedder(according to a psychoacoustic masking level), and to undo thisoperation in the detector. The embedded pattern itself is filtered bythis IIR filter, but the sequence of the pattern is not related to thefilter response.

The article ‘Robust Watermarking for Compressed Video Using Fingerprintsand Its Applications’ by Sooyeun Jung, Dongeun Lee, Seongwon Lee andJoonki Paik (International Journal of Control, Automation, and Systems,vol. 6, no. 6, pp. 794-799, December 2008) discloses a useridentification method at H.264 streaming using watermarking withfingerprints and notes that the trade-off between an infinite-impulseresponse (IIR) filter and a finite-impulse response (FIR) one is thatthe former requires much lower orders for a given desired specification.The paper discloses embedding a watermark in the low spatial frequencyrange. In the detector, they filter out the high frequencies from theimage (which do not contain the watermark) using an infinite impulseresponse filter. Just like in the previously cited paper, the impulseresponse filter is used as pre-processing step to improve the detectionand not for using the detection itself.

US patent application 20040234157 discloses a process where objects aredetected in an image using a spatially variant filter. In one embodimentthe filter is an infinite impulse response difference of Gaussianfilters. The size of the filter is adjusted based on the portion of theimage being processed by the filter. No discussion is made ofimperceptible features. This process is not suitable for watermarking.

The filter comprises at least one feedback connection. This means thatinstead of the conventional FIR filter structures to compute thecorrelation between audio signal and candidate pattern (withoutfeedback), the method is based on the use of infinite impulse response(IIR) filters. In FIR filter structures, the output of the filter onlydepends on the input by means of an inner product of the input signalwith a set of filter coefficients. For IIR filters, on the other hand,the output of the filter depends on both the input as well as the outputgenerated previously. This property is often described by means of afeedback connection in the filter structure. An important advantage ofIIR filter structures is that the desired filter characteristics canoften be realized with a reduced number of coefficients compared to aFIR filter structure, resulting in a reduction in computational andmemory complexity.

In a further embodiment the filter comprises plural all pass delaysections, for example arranged in a cascade. This has the advantage oflow computational complexity. For example, a cascade of 6 all-pass-delaysections requires only 6 delays and 12 multiply-accumulates for eachaudio input sample, which is in many cases significantly less (dependingon the filter length) than frequency-domain correlation (requiringforward and backward FFTs).

In a refinement of this embodiment the filter involves two or more suchcascades in parallel, in which each cascade is configured to detect onespecific pattern. In such application, a sequence of watermark patternscan be retrieved. For example, an embedder may be configured to embedone out of two candidate patterns in each content segment, in which thepattern index is dependent on the value of payload bits. To recoverwhich pattern was embedded in a content segment, the content segment isprocessed by both filter cascades. The output of the cascades issubsequently used to determine which pattern was embedded in the contentsegment.

The two or more cascades may optionally provide semi-orthogonal impulseresponses, and the embedder embeds one of the two cascades. Thisprovides an advantageous construction of the patterns that relate to thepayload. The payload in this embodiment determines which one out of aset of different (inverted) impulse response is used in the contentsegment. The selection of the filter should depend on the payload.

Preferably one or more of the respective impulse responses is modifiedprior to embedding, the modification being dependent on the payload.This embodiment combines two (inverted) impulse responses. While the twopatterns are the same, the payload now determines the relative delaybetween the two responses. As soon as the relative delay is applied, asingle pattern related to the payload is obtained that can be embedded.

Optionally information concerning the relationship between the responsesof the two or more cascades is embedded as the payload. This informationmay concern e.g. a relative delay between the responses or the relativephase or sign of the two or more patterns.

Preferably the impulse response of the filter is truncated at anarbitrary point. In theory the IIR response is of infinite length, hencetruncation is usually appropriate.

The invention further provides systems for implementing the methods ofthe invention and computer program products for causing a computer toexecute the methods of the invention.

BRIEF DESCRIPTION OF THE FIGURES

The invention will now be discussed in more detail with reference to thefigures, in which

FIG. 1 schematically shows a system for embedding a pattern as awatermark into a segment of a content item;

FIG. 2 schematically illustrates a system for detecting the patternrepresenting a payload embedded as a watermark into the content segment;

FIG. 3 shows cascade of 3 all pass-delay sections with gain variable gand delay M; and

FIG. 4 shows an example impulse response of a cascade of 6 allpass-delay sections. In the figures, same reference numbers indicatesame or similar features.

DETAILED DESCRIPTION

FIG. 1 schematically shows a system 100 for embedding a pattern 110 as awatermark into a segment 151 of a content item 150, such as movies,pictures, songs, or radio and television shows. The system 100 to thisend comprises a reading module 110 that reads the content 150 from asource 190, e.g. a local storage medium or a remote location. Anembedding module 120 is configured to modify the content segment 151 toinsert a set of substantially imperceptible features that relate to apattern 121.

The pattern 121 represents a payload that is to be associated with thecontent 150. The payload can be any item of information that is to becarried in the content 150. A popular kind of payload is an identifierof the content 150, or of the originator, rights holder and/ordistributor of the content 150, for example as an alphanumeric name oras symbolic string of characters that is translated to a name and/orother identifying information using an external translation table. Ifsuch a payload is extracted, the identifier can be used to identify theappropriate party.

However, the payload does not have to be the same in all segments; thepayloads of each of the plural segments 151 may and preferably aremutually different. For example the payloads may comprise segmentidentifiers that increase sequentially from one segment to the next. Thepayloads may also comprise time codes, for example relative to aninitial segment to allow a detection of where relatively speaking in thecontent 150 the segment occurs. The time codes may also be absolute andthus allow identification of the time that the segment was distributed,for example its original broadcast time.

A payload may also be a combination of static and changing information.For example, an unchanging identifier of the content 150 and/or itsoriginator or rights holder may be combined with a segment-specificidentifier such as a time code. This allows for each segment theidentification of the content 150, originator or rights holder (or thelike) and the segment-specific information such as its position in thecontent 150 or its time of distribution.

Different methods to represent a payload by two or more pseudo-randomsequences are known. One preferred method is to represent individualpayload bits (or combinations of payload bits) by different (orthogonal)pseudo-random patterns. Another is to embed two pseudo-random patternsin which their relative time delay represents the payload. The resultingsequence of pseudo-random patterns is added to the host signal. Thissummation process can be implemented in various ways, i.e., patterns canbe added consecutively in time or frequency, can be mixed, or can betime and/or frequency interleaved.

Irrespective of the employed method to map a payload to a set ofpseudo-random sequences and the method of combining these sequences intoa single watermark, we can write the resulting watermark as a vectorw[n] with n the sample index that is added to a host signal x[n]:y[n]=x[n]+w[n]  Eq. 4

In many practical cases, the watermark sequence w[n] will be changed inlevel before addition to the host signal x[n] to control theaudibility/robustness trade-off. Moreover, this process is preferablyperformed as a function of both time and frequency to exploit theconcept of auditory masking.

Prior to the embedding module 120 modifying the content segment 151, afilter module 115 determines an impulse response of a filter to be usedfor detecting the pattern 121. More particularly, the filter is aninfinite impulse response filter which can be characterized by asemi-white frequency spectrum and provides a pseudo-random time-domainresponse. Preferably the filter module 115 will truncate this impulseresponse to an arbitrary length, e.g. related to the payload to beembedded.

The embedding module 120 is, in accordance with the present invention,configured for inserting the time-reversed impulse response of thefilter into the segment as the set of imperceptible features. Theinformation concerning the relationship between the responses of the twoor more filters that are embedded is representative of the payload. Thisinformation preferably relates to a relative delay between the responsesor the relative phase or sign of the two or more patterns. The workingof the filter is elaborated upon below with reference to FIG. 3 andfurther.

In an embodiment the filter comprises plural all pass delay sections.This plurality may be arranged in a cascade. Alternatively or inaddition, two or more filters may provide semi-orthogonal impulseresponses. In such a case, the embedding module 120 inserts at least oneof these responses when modifying the content segment 151.

Having modified the content segment 151, the system 100 may repeat themodification operation as described above with further content segments.When the modification operations are all completed, the end result,being watermarked content 195, is provided to an output 199. The output199 can be a storage medium, e.g. a DVD disc to which the content 195 iswritten or recorded. Alternatively the output 199 is a transmissionmedium such as the internet over which the content 195 is distributed tofurther entities.

FIG. 2 schematically illustrates a system 200 for detecting the pattern121 representing a payload embedded as a watermark into a contentsegment 151. This system 200 provides the converse of the system 100.

The system 200 comprises an input 201 at which a segment of thewatermarked content 195 is presented. The input 201 could comprise anetwork connection to gain access to content downloadable from theInternet or similar source, or a connection to television and/or radiobroadcasts. Alternatively the input 201 could be a connection to astorage medium on which content is stored.

The system 200 further comprises a detecting module 230, which isconfigured to detect a set of substantially imperceptible features thatrelate to the pattern 121 in the content segment 151. Detecting thesefeatures allows the pattern 121 to be recovered, which in turn allows adetermination of the payload associated with the content segment 151. Asnoted above, the payload could be unique for each segment 151 or be thesame for all segments part of a content item 150.

In accordance with the invention, the system 200 further comprises afilter module 210, configured for applying to the content segment afilter, more particularly an infinite impulse response filter which canbe characterized by a semi-white frequency spectrum and providing apseudo-random time response.

The filter module 210 is connected to a processing module 220 which isconfigured to process the output of the filter to obtain the pattern 121from which the payload can potentially be reconstructed. Preferablyplural filters are applied to the content segment 151 multiple times,each filter outputting a different candidate pattern. Then theprocessing module 220 takes the response that has the best match withone candidate pattern as the pattern from which the payload can bereconstructed. Said reconstruction is subsequently done at the detectingmodule 230.

In the filter module 210, for one or more candidate payload bits i, eachof the associated sequence of pseudo-random patterns w_(i)[n] iscorrelated with the host signal:

$\begin{matrix}{r_{i} = {\max\limits_{n}\mspace{11mu}{\sum\limits_{t}\;{{y\left\lbrack {t + n} \right\rbrack}{w_{i}\lbrack t\rbrack}}}}} & {{Eq}.\mspace{14mu} 5}\end{matrix}$

Taking the maximum value across n ensures that a potential asynchrony(delay) between candidate pattern w_(i)[n] and host signal is resolved.The pattern w_(i)[n] that results in the highest correlation r_(i) isselected and the associated payload bits are provided as reconstructedpayload:

$\begin{matrix}{f = {\arg{\max\limits_{i}\mspace{11mu}\left( r_{i} \right)}}} & {{Eq}.\mspace{14mu} 6}\end{matrix}$

Preferably, and in line with the spectro-temporal watermark levelmodification during embedding, the host signal is normalized (whitened)before correlation to reduce the variability in correlation due to thehost signal itself.

For large patterns w_(i)[n] (which are often required for sufficientrobustness and transparency), the correlation operation given above isoften performed in the frequency domain to reduce the computationalcomplexityR _(i) [k]=Y[k]W _(i) [k]with Y[k] being the frequency-domain (DFT) representation of y[n] ofsize k:

$\begin{matrix}{{X\lbrack k\rbrack} = {\sum\limits_{n}\;{{x\lbrack n\rbrack}{\exp\left\lbrack {{- 2}\pi\; j\;{nk}\text{/}K} \right\rbrack}}}} & {{Eq}.\mspace{14mu} 7}\end{matrix}$

The value for r_(i) is then found by taking the maximum (peak) ofR_(i)[k] after computing the inverse DFT transform:

$\begin{matrix}{r_{i} = {\max\limits_{n}{\sum\limits_{k}{{R_{i}\lbrack k\rbrack}\;\frac{\exp\left( {2\pi\; j\;\pi\; k\text{/}K} \right)}{K}}}}} & {{Eq}.\mspace{14mu} 8}\end{matrix}$

The result, i.e. the pattern 121 is translated into the payload

FIG. 3 shows cascade of 3 all pass-delay sections with gain variable gand delay M. As noted earlier, the invention is based on the insightthat the correlation operation normally used in watermarking hassimilarities with the convolution operation of applying anfinite-impulse response onto an input signal to obtain a filteredsignal. This filter should be an infinite impulse response filtercharacterized by a semi-white frequency spectrum and provides apseudo-random time-domain response.

One class of filters that is so characterized is a cascade of allpass-delay sections. Because the filter response of an all pass filteris spectrally white, the cascade of such filters will also be spectrallywhite. By varying the gain and/or delay parameters of each allpass-delay section, different pseudo-random sequences can be generated.Furthermore, the number of sections, and the parameters of each sectiondetermine the length of the impulse response of the cascade. An exampleimpulse response of a cascade of 6 all pass-delay sections is shown inFIG. 4. The horizontal axis shows the time, and the vertical axis theimpulse response.

An important benefit of the cascade as detection process is its lowcomputational complexity. A cascade of 6 all pass-delay sectionsrequires only 6 delays and 12 multiply-accumulates for each audio inputsample, which is in many cases significantly less (depending on thefilter length) than frequency-domain correlation (requiring forward andbackward FFTs).

A second benefit relies in the fact that this approach does not need anexhaustive frame-based search for synchronization. Each sample that isprovided as input to the cascade all pass-delay detector will provide anoutput representing the correlation between the impulse response and allformer input samples.

The above provides a description of several useful embodiments thatserve to illustrate and describe the invention. The description is notintended to be an exhaustive description of all possible ways in whichthe invention can be implemented or used. The skilled person will beable to think of many modifications and variations that still rely onthe essential features of the invention as presented in the claims. Inaddition, well-known methods, procedures, components, and circuits havenot been described in detail.

The invention is preferably implemented in a computer program product,i.e. a collection of computer program instructions stored on a computerreadable storage device for execution by a computer. The instructions ofthe present invention may be in any interpretable or executable codemechanism, including but not limited to scripts, interpretable programs,dynamic link libraries (DLLs) or Java classes. The instructions can beprovided as complete executable programs, as modifications to existingprograms or extensions (“plugins”) for existing programs. Moreover,parts of the processing of the present invention may be distributed overmultiple computers or processors for better performance, reliability,and/or cost.

Machine-readable storage devices suitable for storing computer programinstructions include all forms of non-volatile memory, including by wayof example semiconductor memory devices, such as EPROM, EEPROM, andflash memory devices, magnetic disks such as the internal and externalhard disk drives and removable disks, magneto-optical disks and CD-ROMdisks. The computer program product can be distributed on such a storagedevice, or may be offered for download through HTTP, FTP or similarmechanism using a server connected to a network such as the Internet. Tothis end one may connect a server system comprising the storage mediumdiscussed above to a network, and arrange this server for allowing theinstructions to be downloaded to client systems connected directly orindirectly to the network.

When constructing or interpreting the claims, any mention of referencesigns shall not be regarded as a limitation of the claimed feature tothe referenced feature or embodiment. The use of the word “comprising”in the claims does not exclude the presence of other features thanclaimed in a system, product or method implementing the invention. Anyreference to a claim feature in the singular shall not exclude thepresence of a plurality of this feature. The word “means” in a claim canrefer to a single means or to plural means for providing the indicatedfunction.

The invention claimed is:
 1. A method of embedding a watermark patterninto a content segment, the method comprising: modifying the contentsegment to insert a set of features that relate to the watermarkpattern, wherein at least a part of the set of features areimperceptible; determining an impulse response of a filter to be usedfor detecting the watermark pattern; and inserting the time-reversedimpulse response of the filter into the segment as the set of features,wherein the filter is an infinite impulse response filter, wherein theinfinite impulse response filter comprises a semi-white frequencyspectrum and provides a pseudo-random time-domain response.
 2. Themethod of claim 1, wherein the filter comprises a plurality of all passdelay sections, each producing a respective impulse response.
 3. Themethod of claim 2, wherein the plurality is arranged in a cascade. 4.The method of claim 2, wherein two or more filters providesemi-orthogonal impulse responses, and the step of modifying inserts atleast one of these responses.
 5. The method of claim 4, furthercomprising modifying one or more of the respective impulse responsesprior to embedding, the modification being dependent on the payload. 6.The method of claim 5, wherein information concerning the relationshipbetween the responses of the two or more filters that are embedded isrepresentative of the payload.
 7. The method of claim 1, wherein theimpulse response of the filter is truncated at an arbitrary point.
 8. Amethod of detecting a watermark pattern representing a payload embeddedas a watermark into a content segment, the method comprising: detectinga set of features that relate to the watermark pattern in the contentsegment, wherein at least a part of the set of features areimperceptible; applying a filter to the content segment, the filterbeing an infinite impulse response filter; and processing the output ofthe filter to obtain the watermark pattern from which to reconstruct thepayload, wherein the infinite impulse response filter comprises asemi-white frequency spectrum and provides a pseudo-random time-domainresponse.
 9. The method of claim 8, wherein the method comprising:applying a plurality of filters to the content segment multiple times,each filter outputting a different candidate watermark pattern formwhich to reconstruct the payload; and taking the response that has abest match with one candidate watermark pattern as the watermark patternfrom which to reconstruct the payload.
 10. A system for embedding awatermark pattern into a content segment, the system comprising: anembedding module for modifying the content segment to insert a set offeatures that relate to the watermark pattern, wherein at least a partof the set of features are imperceptible; a filter module configured fordetermining an impulse response of a filter to be used for detecting thewatermark pattern, wherein the embedding module is configured to insertthe time-reversed impulse response of the filter into the segment as theset of features, wherein the filter is an infinite impulse responsefilter, and wherein the infinite impulse response filter comprises asemi-white frequency spectrum and provides a pseudo-random time-domainresponse.
 11. A system for detecting a watermark pattern representing apayload embedded as a watermark into a content segment, the systemcomprising: a detecting module for detecting a set of features thatrelate to the watermark pattern in the content segment, wherein at leasta part of the set of features are imperceptible; a filter configured forapplication to the content segment, the filter being an infinite impulseresponse filter; and processing means for processing the output of thefilter to obtain the watermark pattern from which the payload canpotentially be reconstructed, wherein the infinite impulse responsefilter comprises a semi-white frequency spectrum and provides apseudo-random time-domain response.
 12. The method of claim 6, whereinthe information relates to a relative delay between the responses or therelative phase or sign of the two or more watermark patterns.